Layouts: a viewport on first opening (LAYOUTCREATEVIEWPORT)

A layout opened for the first time - one whose sheet viewport had to be
made, or one created in this session - gets a viewport centred on the
printable area, 0.8 of its width and height, zoomed to the model's
extents, while LAYOUTCREATEVIEWPORT is on. The profile variable takes
ON/OFF or 1/0 and is kept with the settings.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/app/control/sheets.rs

@@ -136,7 +136,6 @@ impl OpenCADStudio {
         self.push_undo_snapshot(i, "LAYOUTCREATE");
         self.tabs[i]
             .scene
-            .document
             .add_layout(&name)
             .map_err(|e| failure("layout_failed", e))?;
         let plot_style = self